1. Field of the Invention
This invention relates in general to analog-to-digital (A/D) converters and, more particularly, to a new architecture for oversampled delta-sigma (.DELTA.-.SIGMA.) A/D conversion of high frequency, narrow band signals using cascaded low order stages to obtain high overall order of noise shaping.
2. Description of the Prior Art
Prior art single-loop bandpass delta-sigma (.DELTA.-.SIGMA.) modulation is described by R. Schreier and M. Snelgrove in "Bandpass Sigma Delta Modulation," Electronics Letters, Vol. 25, 9th Nov. 1989, pp. 1560-1561. Z.X. Zhang, G.C. Temes and Z. Czarnul in "Bandpass .DELTA..SIGMA. A/D Converter Using Two-Path Multibit Structure", EIectronics Letters, Vol. 27, 24th Oct. 1991, pp. 2008-2009, citing Schreier et al., supra, describe a modification of the basic structure using N-path switched capacitor circuits. Subsequently, R. Schreier, G.C. Temes, A.G. Yesilyurt, Z.X. Zhang, Z. Czarnul, and A. Hairapetian in "Multibit Bandpass Delta-Sigma Modulators Using N-Path Structures", IEEE International Symposium on Circuits and Systems, May 1992, pp. 593-596, describe simulation results of two switched-capacitor circuits for use in a multi-bit bandpass sigma-delta modulator.
Typically, cascaded low-order delta-sigma stages have been used to implement higher order delta-sigma modulators for A/D conversion of low-pass signals. See, for example, D.B. Ribner, "A Comparison of Modulator Networks for High-Order Oversampled Sigma-Delta Analog-to-Digital Converters," IEEE Trans. Circuits and Systems, Vol. CAS-38, No. 2, pp. 145-159, Feb. 1991. Those cascades comprise combinations of first and second-order individual stages and thereby avoid the stability problem prone to single-loop modulators of third-order and higher. See also D.E. Ribner U.S. Pat. Nos. 5,084,702, 5,103,229, and 5,148,166, all assigned to the instant assignee, for further background on plural-order sigma-delta analog-to-digital converters. U.S. Pat. Nos. 5,084,702, 5,103,229 and 5,148,166 are hereby incorporated by reference.